Migmatite is a composite rock found in medium and high-grade metamorphic environments, commonly within Precambrian cratonic blocks. It consists of two or more constituents often layered repetitively: one layer is an older metamorphic rock that was reconstituted subsequently by partial melting ("neosome"), while the alternate layer has a pegmatitic, aplitic, granitic or generally plutonic appearance ("paleosome"). Commonly, migmatites occur below deformed metamorphic rocks that represent the base of eroded mountain chains.
Ptygmatic folding in migmatite on Naissaar Island, Estonia
Migmatite on the coast of Saaremaa, Estonia
Intricately-folded migmatite from near Geirangerfjord, Norway
Cliff section through near-vertically dipping ptygmatically folded migmatites
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than 150 to 200 °C and, often, elevated pressure of 100 megapascals (1,000 bar) or more, causing profound physical or chemical changes. During this process, the rock remains mostly in the solid state, but gradually recrystallizes to a new texture or mineral composition. The protolith may be an igneous, sedimentary, or existing metamorphic rock.
Quartzite, a type of metamorphic rock
Metamorphic rock, deformed during the Variscan orogeny, at Vall de Cardós, Lérida, Spain
Amphibolite formed by metamorphism of basalt
Metamorphic rock containing staurolite and almandine garnet